A Ni—Cr alloy and a Fe—Al—Cr alloy are widely used as a metal-based electrical-resistance heat-generation element, and their critical heat-resistant temperatures are 1100° C. and 1250° C., respectively. Platinum or platinum alloy having heat/corrosion resistances and excellent workability is used as a material of an electrical-resistance heat-generation element for various analytical instruments or the like, capable of precisely controlling the temperature thereof in a temperature range up to 1600° C.
However, these elements have disadvantages, such as reduction in thickness due to oxidative wear caused by a high-temperature oxidation atmosphere, embrittlement caused by a reduction atmosphere containing a carbon compound, and sulfur corrosion caused by a sulfur-containing atmosphere (hydrogen sulfide, sulfur dioxide, etc.).
There has also been known an electrical-resistance heat-generation element made of a refractory metal, such as tungsten or tantalum, having more excellent heat resistance, which is usable in a temperature range of room temperature to 2000° C. or more. However, this element has to be limitedly used in a high-vacuum environment due to its poor oxidation resistance. Moreover, the element made of a refractory metal cannot be used in a harsh environment, because the occurrence of a defect in its surface layer leads to a catastrophic oxidation in the inside thereof. In order to allow such a refractory metal to be used even in an oxidation atmosphere for a long period of time, an electrical-resistance heat-generation element has been proposed which comprises a refractory-metal core and a zirconia coating film formed on the core (Japanese Patent Laid-Open Publication No. 05-299156).
A silicon carbide heat-generation element and a molybdenum disilicide heat-generation element are known as a nonmetal-based heat-generation element, and used in oxidation atmospheres in temperature ranges up to 1650° C. and 1750° C., respectively. However, each of the elements made of such a brittle material has disadvantages of poor workability and low thermal shock resistance. Moreover, the use of a carbon-based heat-generation element in an oxidation atmosphere is restricted due to oxidative wear.
A rhenium metal has a high melting point next to that of tungsten, and an electrical resistance which is 2 to 4 times greater than that of a platinum-group metal or a refractory metal. Such high melting point and electrical resistance are desirable properties as a material of a heat-generation element, particularly of a foil strip or an extra fine wire, and thus the rhenium metal has great potential as a material of a resistance heat-generation element to be used at an ultrahigh temperature. However, the rhenium metal has low oxidation resistance, and poor workability due to its brittleness.